OTU: pulmonary oxygen toxicity
The oxygen dose your dive puts on your lungs, and what you have left before your daily limit.
Your dive
Mix, depth and time. Add your stops for the full dive.
Your day
What you have already breathed, and the threshold not to exceed.
Your limit
A single day of diving, with no oxygen the day before or after.
Oxygen dose of the dive
58OTU
Within the limit
In recreational diving, mind your CNS % first: it usually limits before OTU.
Reference: OTU per minute by ppO2
Your bottom ppO2 is highlighted. Below 0.5 bar, oxygen does not count for the lungs.
| ppO2 (bar) | OTU/min | For 850 OTU |
|---|---|---|
| 0.50 | 0.00 | ∞ |
| 0.55 | 0.15 | 96 h 30 |
| 0.60 | 0.26 | 54 h 10 |
| 0.65 | 0.37 | 38 h 38 |
| 0.70 | 0.47 | 30 h 24 |
| 0.75 | 0.56 | 25 h 14 |
| 0.80 | 0.65 | 21 h 41 |
| 0.85 | 0.74 | 19 h 04 |
| 0.90 | 0.83 | 17 h 03 |
| 0.95 | 0.92 | 15 h 28 |
| 1.00 | 1.00 | 14 h 10 |
| 1.05 | 1.08 | 13 h 05 |
| 1.10 | 1.16 | 12 h 10 |
| 1.15 | 1.24 | 11 h 23 |
| 1.20 | 1.32 | 10 h 42 |
| 1.25 | 1.40 | 10 h 06 |
| 1.30 | 1.48 | 9 h 34 |
| 1.35 | 1.56 | 9 h 06 |
| 1.40 | 1.63 | 8 h 40 |
| 1.45 | 1.71 | 8 h 17 |
| 1.50 | 1.78 | 7 h 57 |
| 1.55 | 1.86 | 7 h 38 |
| 1.60 | 1.93 | 7 h 20 |
How it is calculated
1. ppO2
Absolute pressure (sea water, 10 m = 1 bar) multiplied by the oxygen fraction.
P = 1 + depth ÷ 10 · ppO2 = FO2 × P
2. OTU
1 OTU is 1 minute of pure oxygen at 1 bar. Below 0.5 bar, it counts as 0. Above, the dose rises faster than the ppO2. This is the UPTD model of Bardin and Lambertsen (1970), used in the NOAA tables and most computers.
OTU = t × ((ppO2 − 0.5) ÷ 0.5)^(5/6)
Descent and ascent
ppO2 changes continuously. The tool then integrates the formula exactly (Erik Baker’s method, used by Subsurface). With the option off: classic course method, all the time is counted at max depth and the ascent is not counted. The difference is usually a few OTU.
OTU = 3/11 × t ÷ (ppO2f − ppO2i) × [xf^(11/6) − xi^(11/6)]
3. Max bottom time
The bottom time (descent included) that would bring your daily dose exactly to the limit, keeping your stops and what you have already done.
t max = (limit − dose outside the bottom) ÷ OTU per minute on the bottom
4. Vital capacity loss
Median drop in vital capacity (half of divers lose less, half more), interpolated on the points published by Bardin and Lambertsen from Clark’s measurements: 615 OTU ≈ 2 %, 825 ≈ 4 %, 1035 ≈ 6 %, 1230 ≈ 8 %, 1425 ≈ 10 %, 1815 ≈ 15 %, 2190 ≈ 20 %. Below 615 OTU, the drop cannot be reliably measured.
5. Limits
One day: 850 OTU. Several days (NOAA REPEX method): the allowed dose drops each day, 850, 700, 620, 525, 460, 420, 380, 350, 330, 310, then 300 OTU per day, with a cumulative cap (850, 1400, 1860, 2100, 2300, 2520, 2660, 2800, 2970, 3100, then +300 per day). The tool keeps the smaller of the two. Cautious: 615 OTU, often quoted as the maximum for a single dive (≈ 2 % vital capacity).
6. What this model cannot do
It does not compute recovery: the counter is reset each day, like most computers. REPEX limits were never scientifically validated in divers, and the model fits high ppO2 poorly. For professional diving, DMAC has recommended another index (ESOT) since 2025. In recreational diving, CNS % usually limits before OTU. Fresh water and altitude are not taken into account (small effect).
A learning tool. It does not replace your training, your computer or your agency’s procedures.
Understanding OTU
OTU, the lungs’ oxygen dose
Breathing oxygen under pressure slowly irritates the lungs. The OTU (Oxygen Tolerance Unit) measures that dose: 1 OTU is one minute of pure oxygen at 1 bar. Below 0.5 bar ppO2, nothing counts; above, the dose climbs faster than the ppO2. The tool adds up each segment of your dive and compares it with your daily limit. For your ppO2 at each depth, use the PpO2 tool.
REPEX limits: 850 OTU a day, less over several days
NOAA allows 850 OTU on a single day. Over several days in a row (REPEX method), the allowed dose drops each day and a cumulative cap applies: 1400 OTU over two days, 1860 over three. The tool keeps the smaller of the two. Cautious mode keeps 615 OTU, the threshold where vital capacity drops by about 2 %.
OTU or CNS: which one limits?
CNS % tracks oxygen toxicity for the nervous system, OTU for the lungs. In recreational diving, over one or two dives a day, CNS almost always limits first: work it out with the CNS % tool. OTU becomes decisive on multi-day trips, long oxygen stops and technical diving.
Frequently asked questions
How do you work out the OTU of a dive?
OTU = time × ((ppO2 − 0.5) ÷ 0.5)^(5/6) for each segment, then add them up. Advanced nitrox course example: EAN30 at 30 m for 40 min + EAN80 at 3 m for 19 min = about 73 OTU.
What is the maximum OTU per day?
850 OTU on a single day (NOAA). Over several days, the REPEX limit drops: 700 on day two, 620 on day three, down to 300 per day, with a cumulative cap.
How many OTU per minute at 1.4 bar?
1.63 OTU per minute. At 1.0 bar it is 1 OTU per minute; at 1.6 bar, 1.93; below 0.5 bar, 0.
What does vital capacity loss mean?
It is the median drop in the volume of air your lungs can move, measured after exposure. About 2 % at 615 OTU, 4 % at 825 OTU. It is an order of magnitude, not a personal measurement.